Rifling of Cannon Projectiles

Rifling, by imparting a spin to the projectile as it
travels along the spiral grooves in the bore, permits the use of a long
projectile and ensures its flight point first, with great increase in
accuracy. The longer projectile, being both heavier and more streamlined
than round shot of the same caliber, also has a greater striking energy.

Though Benjamin Robins was probably the first to give
sound reasons, the fact that rifling was helpful had been known a long
time. A 1542 barrel at England, has six fine spiral grooves in the bore.
Straight grooving had been applied to small arms as early as 1480, and
during the 1500's straight grooving of musket bores was extensively
practiced, Probably, rifling evolved from the early observation of the
feathers on an arrow—and from the practical results of cutting channels
in a musket, originally to reduce fouling, then because it was found to
improve accuracy of the shot. Rifled small-arm efficiency was clearly
shown at Kings Mountain during the American Revolution.

In spite of earlier experiments, however, it was not
until the 1840's that attempts to rifle cannon could be called
successful. In 1846, Major Cavelli in Italy and Baron Wahrendorff in
Sweden independently produced rifled iron breech-loading cannon. The
Cavelli gun had two spiral grooves into which fitted the 1/4-inch
projecting lugs of a long projectile (fig. 12a). Other attempts at what
might be called rifling were Lancaster's elliptical-bore gun and the
later development of a spiraling hexagonal-bore by Joseph Whitworth
(fig. 12b). The English Whitworth was used by Confederate artillery. It
was an efficient piece, though subject to easy fouling that made it
dangerous.

Then, in 1855, Britain's Sir William Armstrong
designed a rifled breech-loader that included so many improvements as to
be revolutionary. This gun was rifled with a large number of grooves and
fired lead-coated projectiles. Much of its success, however, was due to
the built-up construction: hoops were shrunk on over the tube, with the
fibers of the metal running in the directions most suitable for
strength. Several United States muzzle-loading rifles of built-up
construction were produced about the same time as the Armstrong and
included the Chambers (1849), the Treadwell (1855), and the well-known
Parrott of 1861 (figs. 12e and 13).

The German Krupp rifle had an especially successful
breech mechanism. It was not a built-up gun, but depended on superior
crucible steel for its strength. Cast-steel had been tried as a gunmetal
during the sixteenth and seventeenth centuries, but metallurgical
knowledge of the early days could not produce sound castings. Steel was
also used in other mid-nineteenth century rifles, such as the United
States Wiard gun and the British Blakely, with its swollen, cast-iron
breech hoop. Fort Pulaski National Monument, near Savannah, Ga., has a
fine example of a 24-pounder Blakely used by the Confederates in the
1862 defense of the fort.

The United States began intensive experimentation
with rifled cannon late in the 1850's, and a few rifled pieces were made
by the South Boston Iron Foundry and also by the West Point Foundry at
Cold Spring, N. Y. The first appearance of rifles in any quantity,
however, was near the outset of the 1861 hostilities, when the Federal
artillery was equipped with 300 wrought-iron 3-inch guns (fig. 14e).
This "12-pounder," which fired a 10-pound projectile, was made by
wrapping sheets of boiler iron around a mandrel. The cylinder thus
formed was heated and passed through the rolls for welding, then cooled,
bored, turned, and rifled. It remained in service until about 1900.
Another rifle giving good results was the cast-iron 4-1/2-inch siege
gun. This piece was cast solid, then bored, turned, and rifled.
Uncertainty of strength, a characteristic of cast iron, caused its later
abandonment.

The United States rifle that was most effective in
siege work was the invention of Robert P. Parrott. His cast-iron guns
(fig. 13), many of which are seen today in the battlefield parks, are
easily recognized by the heavy wrought-iron jacket reinforcing the
breech. The jacket was made by coiling a bar over the mandrel in a
spiral, then hammering the coils into a welded cylinder. The cylinder
was bored and shrunk on the gun. Parrotts were founded in 10-, 20-, 30-,
60-, 100-, 200-, and 300-pounder calibers, one foundry making 1,700 of
them during the Civil War.

FIGURE 13—PARROTT 10-POUNDER RIFLE (1864).

All nations, of course, had large stocks of
smoothbores on hand, and various methods were devised to make rifles out
of them. The U. S. Ordnance Board, for instance, believed the conversion
simply involved cutting grooves in the bore, right at the forts or
arsenals where the guns were. In 1860, half of the United States
artillery was scheduled for conversion. As a result, a number of old
smoothbores were rebored to fire rifle projectiles of the various
patents which preceded the modern copper rotating band (fig. 12c, d, f).
Under the James patent (fig. 12c) the weight of metal thrown by a cannon
was virtually doubled; converted 24-, 32- and 42-pounders fired
elongated shot classed respectively as 48-, 64-, and 84-pound
projectiles. After the siege of Fort Pulaski, Federal Gen. Q. A.
Gillmore praised the 84-pounder and declared "no better piece for
breaching can be desired," but experience soon proved the heavier
projectiles caused increased pressures which converted guns could not
withstand for long.

The early United States rifles had a muzzle velocity
about the same as the smoothbore, but whereas the round shot of the
smoothbore lost speed so rapidly that at 2,000 yards its striking
velocity was only about a third of the muzzle velocity, the more
streamlined rifle projectile lost speed very slowly. But the rifle had
to be served more carefully than the smoothbore. Rifling grooves were
cleaned with a moist sponge, and sometimes oiled with another sponge.
Lead-coated projectiles like the James, which tended to foul the grooves
of the piece, made it necessary to scrape the rifle grooves after every
half dozen shots, although guns using brass-banded projectiles did not
require the extra operation. With all muzzle-loading rifles, the
projectile had to be pushed close home to the powder charge; otherwise,
the blast would not fully expand its rotating band, the projectile would
not take the grooves, and would "tumble" after leaving the gun, to the
utter loss of range and accuracy. Incidentally, gunners had to "run out"
(push the gun into firing position) both smoothbore and rifled
muzzle-loaders carefully. A sudden stop might make the shot start
forward as much as 2 feet.

When the U. S. Ordnance Board recommended the
conversion to rifles, it also recommended that all large caliber iron
guns be manufactured on the method perfected by Capt. T. J. Rodman,
which involved casting the gun around a water-cooled core. The inner
walls of the gun thus solidified first, were compressed by the
contraction of the outer metal as it cooled down more slowly, and had
much greater strength to resist explosion of the charge. The Rodman
smoothbore, founded in 8-, 10-, 15-, and 20-inch calibers, was the best
cast-iron ordinance of its time (fig. 14f). The 20-inch gun, produced in
1864, fired a 1,080-pound shot. The 15-incher was retained in service
through the rest of the century, and these monsters are still to be seen
at Fort McHenry National Monument and Historic Shrine; on the ramparts
of Fort Jefferson, in the national monument of that name, in the Dry
Tortugas Islands; and at Forts Moultrie, Foote, and Massachusetts—all
areas administered by the National Park Service.